Variable pulley transmissions for transferring torque from an input shaft to an output shaft have been used for some time. In such arrangements a first pulley is mounted on the input shaft, and this pulley has at least one flange axially movable relative to its other flange to change the effective pulley diameter. A second, similarly adjustable pulley is mounted on the output shaft. A flexible belt intercouples the two pulleys to transfer torque between them. As the effective diameter of one pulley is changed, and simultaneously the effective diameter of the other pulley is changed in the opposite direction, the drive ratio between the input and output shafts is adjusted in a smooth, continuous manner.
For several decades automotive engineers have recognized that the maximum operating efficiency of the engine could be achieved if the transmission could be controlled by adjusting to different loads and speed ratios such that the engine is maintained at its maximum efficiency point. This is not possible with a conventional geared transmission in which the drive ratio is adjusted in discrete steps, rather than continuously. Accordingly, efforts have been directed to the use of a continuously variable transmission of the type described above. This has resulted in the production and marketing in Europe of the Daf passenger car, using a flexible rubber belt in such a continuously variable transmission (CVT). Such a belt is subject to wear by reason of the torque it must handle, and operates under severe temperature, vibration and other adverse conditions. To improve the belt life, efforts have been channeled to produce a flexible belt of metal, and some of these efforts are described in the patent literature.
One metal belt approach was a central indexing strand composed of a plurality of nested metal bands on which are threaded a plurality of generally trapezoidal or V-shaped (when viewed from the front) metal load or drive blocks, the blocks being longitudinally slidable along the indexing strand during operation. Because the individual blocks are in compression as they transfer torque between the pulleys, this type of flexible belt is frequency termed a "push" belt. It is relatively expensive to manufacture because the metal bands must be matched for their nested assembly, the load blocks must be stamped and finished, then the blocks must be assembled onto the strand of nested bands. An example of this type of push belt is described in U.S. Pat. No. 3,720,112.
Another type of flexible metal utilizes a chain as the indexing strand, in lieu of the more expensive nested band array. Hence such a belt is generally termed a "chain-belt". The chain link sets are connected to cylindrical pins, or other suitable pivot means. The drive blocks are generally trapezoidal or V-shaped and are positioned between a pair of consecutive pivot members. The drive or load blocks in a chain-belt have little, if any, longitudinal movement with respect to the chain. The load blocks generally transmit the load to the chain and the pivot members. This type of chain-belt is termed a "pull" belt, and is much less expensive to manufacture than the "push" belt before described, because the chain links and the load blocks are stamped from sheet metal and the parts are assembled by automatic machinery. There is no need to match band lengths required for nesting the endless bands for the push type belt. An example of a pull type chain-belt is described in U.S. Pat. No. 4,313,730.
As shown in the latter patent, each load block of an assembly defines a central window through which links pass, i.e., the load blocks surround the sets of links. For low torque applications, a relatively narrow load block (and chain) can be used. As the torque requirements are increased, wider chain is required which also requires wider load blocks. Because of the loading of the blocks, at least the bottom span of the block is stressed and at times deflects and twists, leading to block failure. The present invention is particularly directed to improve load blocks for a pull type chain-belt, to enable the chain-belt to transfer a higher torque between the pulleys than is possible with known arrangements.